FEC in cognitive multi-user OFDMA

ABSTRACT

A multiuser scheme allowing for a number of users, sets of user, or carriers to share one or more channels is provided. In the invention, the available channel bandwidth is subdivided into a number of equal-bandwidth subchannels according to standard OFDM practice. A transmitter transmits data on a set of OFDM subchannels that need not be contiguous in the spectrum or belong to the same OFDM channel. A receiver receives and decodes the data and detects errors on subchannels. The receiver then broadcasts the identity of those subchannels on which the error rate exceeds a specific threshold, and the transmitter may select different subchannels for transmission based on this information.

BACKGROUND OF INVENTION

1 . Field of Invention

The invention relates generally to wireless communication and moreparticularly to a system for selecting OFDM subchannels.

2. Discussion of Related Art

Frequency Division Multiplexing (FDM) is a well known process by whichmultiple signals are modulated on different frequency carrier waves. FDMhas been used for decades in radio and television broadcast. Radio andtelevision signals are sent and received on different frequencies, eachcorresponding to a different “channel.”

Orthogonal Frequency Division Multiplexing (OFDM) has also been known inthe art at least since the late 1960's. In OFDM, a single transmittertransmits on multiple different orthogonal frequencies simultaneously.Orthogonal frequencies are frequencies that are independent with respectto the relative phase relationship between the frequencies. In OFDM, theavailable bandwidth is subdivided into a number of equal-bandwidth“subchannels.” OFDM is advantageous for broadband wireless communicationbecause it reduces the detrimental effect of multipath interference,ultimately permitting reliable data transmission at higher throughput.OFDM is also known as Discrete Multitone Modulation (DMT). OFDM isemployed in many Standards used today for wireless communication. Forexample, both the IEEE 802.11a wireless LAN Standard and the 802.11gwireless LAN Standard rely on an implementation of OFDM for signaltransmission. The next generation Wi-Fi (802.11n) and UWB also use OFDM.One early reference describing OFDM is R. W. Chang, Synthesis ofband-limited orthogonal signals for multi-channel data transmission,Bell System Technical Journal (46), 1775-1796 (1966).

OFDM thus functions by breaking one high speed data stream into a numberof lower-speed data streams, which are then transmitted in parallel(i.e., simultaneously). Each lower speed stream is used to modulate asubcarrier. This creates a “multi-carrier” transmission by dividing awide frequency band (or channel) into a number of narrower frequencybands (or subchannels), each modulated with a signal stream. By sendingmultiple signal streams simultaneously, each at a lower rate,interference such as multipath or Raleigh fading can be attenuated oreliminated without decreasing the overall rate of transmission.

Orthogonal Frequency Division Multiple Access (OFDMA) is an improvementon OFDM. In OFDMA, different sets of subchannels are assigned todifferent users. OFDMA is employed today in the DVB-RCT specificationsfor terrestrial interactive TV networks and in the IEEE 802.16a and IEEE802.16e (mobile WiMAX) specifications for broadband wireless accessnetworks. OFDMA was described in H. Sari and G. Karam, “OrthogonalFrequency-Division Multiple Access and its Application to CATVNetworks,” European Transactions on Telecommunications & RelatedTechnologies (ETT), Vol. 9, No. 6, pp. 507-516, November-December 1998.OFDMA is also known as Multi-user OFDM.

Cognitive radio is a system used for wireless communication in whichtransmitters and receivers can alter communications parameters based ona variety of factors. A nonexclusive list of these factors includes thenature of the communication being transmitted, the availability oflicensed or unlicensed frequencies, user behavior, network state, noiseor other interference at particular frequencies, and detection of otherusers of bandwidth. Cognitive radio is discussed generally in J. Mitola,III and G. Q. Maguire, Jr., “Cognitive Radio: Making Software RadiosMore Personal,” IEEE Personal Communications, 6(4):13-18, August 1999.

Error correction coding or forward error correction is a method to checkthat signals have been correctly received and correct errors intransmission when they occur. Generally error correction coding operatesby adding some form of redundant data to a message. Different errorcorrection coding schemes tolerate different levels of errortransmissions without requiring any data to be retransmitted.

SUMMARY OF INVENTION

This Summary provides an illustrative context for aspects of theinvention, in a simplified form. It is not intended to be used todetermine the scope of the claimed subject matter. Aspects of theinvention are described more fully below in the Detailed Description.

In the claimed invention, a receiver performs error correction decodingon received packets, and uses that information to locate the OFDMsubchannels causing errors and thus determine which subchannels are bestsuited for use.

Described herein are systems and methods for the implementation of amultiuser scheme allowing for a number of users, sets of user, orcarriers to share one or more channels. In some embodiments, theavailable channel bandwidth is subdivided into a number ofequal-bandwidth subchannels according to standard OFDM practice. Thetransmitter is informed by an application that it needs to transmit dataa particular rate. The transmitter determines the minimum number ofsubchannels and maximum interference plus noise power threshold for eachsubchannel necessary to achieve that data rate and selects a set ofsubchannels matching those requirements. The subchannels need not becontiguous in the spectrum or belong to the same channel. Once thetransmitter has selected the required number of subchannels, it beginstransmitting simultaneously on those subchannels across the entirebandwidth used by those subchannels.

The receiver then performs error correction on the interleaved receivedpackets. The error correction decoder will first locate the errors, andthus determine which subchannels cause errors. If the number of bits inerror in a given subchannel exceeds a threshold, then the channel islabeled as bad and another channel is used. In various embodiments,either a “hard decision” or a “soft decision” can be made as to whichchannels cause the most errors. In some embodiments, the receiver thenbroadcasts a vector indicating which channels are bad and which channelsare good, which the sender (or the network) can then use for subchannelselection.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings are not intended to be drawn to scale. In thedrawings, each identical or nearly identical component that isillustrated in various figures is represented by a like numeral. Forpurposes of clarity, not every component may be labeled in everydrawing. In the drawings:

FIG. 1 is a spectrum diagram showing the subdivision of the channelbandwidth to be used into several subchannels of equal width.

FIG. 2 is a block diagram of a multi-carrier OFDM digital communicationsystem.

FIG. 3 is a flow diagram illustrating one embodiment of the invention.

FIG. 4 is a flow diagram illustrating one embodiment of the invention.

FIG. 5 is a diagram of a system that implements some aspects of theinvention.

DETAILED DESCRIPTION

This invention relates to a novel use of forward error correction toselect subchannels for OFDMA transmission. Cognitive selection of OFDMAsubchannels is described in copending application Ser. No. 11/410,969 toHassan et al. In embodiments of the claimed invention, the selection ofOFDM subchannels incorporates error coding information as describedbelow. The system thus permits several users or sets of users can sharethe same bandwidth more efficiently. The invention may be implemented inhardware or software, or some combination thereof. Embodiments include asystem, a method, and instructions stored in a computer-readable medium.

Computer readable media can be any available media that can be accessedby a computer. By way of example, and not limitation, computer readablemedia may comprise computer storage media and communication media.Computer storage media includes volatile and nonvolatile, removable andnon-removable media implemented in any method or technology for storageof information such as computer readable instructions, data structures,program modules or other data. Computer storage media includes, but isnot limited to, RAM, ROM, EEPROM, flash memory or other memorytechnology, CD-ROM, digital versatile disks (DVD) or other opticalstorage, magnetic cassettes, magnetic tape, magnetic disk storage orother magnetic storage devices, other types of volatile and non-volatilememory, any other medium which can be used to store the desiredinformation and which can accessed by a computer, and any suitablecombination of the foregoing.

The computer-readable media may be transportable such that theinstructions stored thereon can be loaded onto any suitable computersystem resource to implement the aspects of the present inventiondiscussed herein. In addition, it should be appreciated that theinstructions stored on the computer-readable medium, described above,are not limited to instructions embodied as part of an applicationprogram running on a host computer. Rather, the instructions may beembodied as any type of computer code (e.g., software or microcode) thatcan be employed to program a processor to implement the aspects of thepresent invention discussed below.

This invention is not limited in its application to the details ofconstruction and the arrangement of components set forth in thefollowing description or illustrated in the drawings. The invention iscapable of other embodiments and of being practiced or of being carriedout in various ways. Also, the phraseology and terminology used hereinis for the purpose of description and should not be regarded aslimiting. The use of “including,” “comprising,” or “having,”“containing,” “involving,” and variations thereof herein, is meant toencompass the items listed thereafter and equivalents thereof as well asadditional items.

As shown in FIG. 1, in OFDM, the available channel bandwidth W issubdivided into a number of equal-bandwidth subchannels. Each subchannelis sufficiently narrow so that the frequency response characteristics ofthe subchannel are nearly ideal. The number of subchannels is the totalavailable bandwidth divided by the bandwidth of each subchannel. Thenumber of subchannels K can thus be expressed as:

$K = \frac{W}{\Delta\; f}$

Each subchannel k has an associated carrier wave. This carrier wave canbe expressed as:x _(k)(t)=sin 2πƒ_(k) t

Where x_(k)(t) is the carrier wave for subchannel k as a function oftime t. ƒ_(k) is the mid-frequency of subchannel k, and k ranges from 0to K-1.

The symbol rate 1/T is set for each subchannel to be equal to theseparation Δƒ of adjacent subcarriers. The subcarriers will thus beorthogonal over the symbol interval T, independent of the relative phaserelationship between subcarriers. This relationship can be expressed as:

∫₀^(T)sin (2π f_(k)t + ϕ_(k))sin (2π f_(j)t + ϕ_(j))𝕕t = 0

Where ƒ_(k)−ƒ_(j)=n/T, n=1, 2, . . . , independent of the values of thephases Φ_(k) and Φ_(j).

In an OFDM system, the symbol rate on each subchannel can be reducedrelative to the symbol rate on a single carrier system that employs theentire bandwidth W and transmits data at the same rate as the OFDMsystem. Hence, the symbol interval T (the inverse of the symbol rate) inthe OFDM system can be expressed as:T=KT_(s)

Where T_(s) is the symbol interval of a single-carrier system employingthe entire bandwidth W and transmitting data at the same rate as theOFDM system. For example, if the symbol rate across the entire bandwidthfor one channel is 72 million symbols per second, and the channel isdivided into 48 subchannels, each subchannel would only need to carry1.5 million symbols per second to achieve the same total data rate. Thislower symbol rate reduces inter-symbol interference and thus mitigatesthe effects of multipath fading. Accordingly, OFDM provides for superiorlink quality and robustness of communication.

In an OFDM system, the transmitter receives input data in the frequencydomain and converts it to a time domain signal. A carrier wave ismodulated by the time domain signal for wireless transmission. Thereceiver receives the signal, demodulates the wave, and converts thesignal back to the frequency domain for further processing.

A simplified OFDM system is illustrated in FIG. 2. In the illustratedembodiment, the input data stream 201 is provided by the application tothe OFDM transmitter 200. In a standard TCP/IP communications stack,this data would be received at the physical layer or data link layer;however, the invention is not limited to any particular source of dataor mechanism for providing the data to the transmitter, and could beimplemented in hardware or software, and at various layers of thenetwork stack. The input data stream 201 is received by aserial-to-parallel buffer 202. The serial-to-parallel buffer 202 breaksthe serial data stream up into several parallel data streams. The numberof parallel data streams is normally equal to the number of subchannelsselected for OFDM broadcast, or K as used above. In other systemsconsistent with some embodiments of the invention, more than K parallelstreams may be used. The novel process of selecting channels for OFDMbroadcast claimed in this patent is discussed below.

In one embodiment, the serial-to-parallel buffer 202 divides theinformation sequence received from input data 201 into frames of B_(f)bits. The B_(f) bits in each frame are parsed into K groups, where theith group is assigned b_(i) bits. This relationship may be expressed as:

${\sum\limits_{i = 1}^{K}b_{i}} = B_{f}$

Each of the parallel data streams generated by the serial-to-parallelbuffer 202 is then sent to a multicarrier modulator 203. Themulticarrier modulator 203 modulates each selected subcarrier with eachof the parallel data streams. The multicarrier modulator 203 can beefficiently implemented by use of the Inverse Fast Fourier Transformalgorithm to compute the time domain signal, although any algorithm maybe used that converts a frequency domain signal to a time domain signal.

The multicarrier modulator 203 may use any modulation scheme to modulateeach of the incoming data streams. In a preferred embodiment, thesignals are modulated with quadrature amplitude modulation (QAM). AnyQAM constellation may be used.

For example, the modulator may use 16-QAM, 64-QAM, 128-QAM or 256-QAM. Amodulation scheme may be selected based on the required data rate, theavailable subchannels, the noise on each subchannel, or other factors.

In this example, the multicarrier modulator 203 thus generates Kindependent QAM subchannels, where the symbol rate for each subchannelis 1/T and the signal in each subchannel has a distinct QAMconstellation. According to this example, the number of signal pointsfor the ith subchannel can be expressed as:M_(i)=2^(b) ^(i)

The complex-valued signal points corresponding to the informationsignals on each of the K subchannels can be represented as X_(k), wherek=0, 1, . . . , K-1. These symbols X_(k) represent the values of theDiscrete Fourier Transform of a multicarrier OFDM signal x(t), where themodulation on each subcarrier is QAM. Since x(t) must be a real-valuedsignal, its N-point Discrete Fourier Transform X_(k) must satisfy thesymmetry property. Therefore, the system creates N=2K symbols from Kinformation symbols by defining:X _(N-K) =X* _(K) , k=1,2, . . . , K-1X′ ₀ =Re(X ₀)X _(N) =Im(X ₀)

Here X₀ is split into two parts, both of which are real. The newsequence of symbols can be expressed as X′_(k), where k=0, 1, . . . ,N-1. The N-point Inverse Direct Fourier Transform for each subchannelx_(n) can thus be expressed as:

$x_{n} = {\frac{1}{\sqrt{N}}{\sum\limits_{k = 0}^{N - 1}{X_{k}^{\prime}{\exp\left( {{j2\pi}\;{{nk}/N}} \right)}}}}$n = 0, 1, …  , N − 1

In this equation,

$\frac{1}{\sqrt{N}}$is a scale factor. The sequence x_(n) where 0<=n<=N-1 thus correspondsto samples of the multicarrier OFDM signal x(t), consisting of Ksubcarriers.

A cyclic prefix, which acts a guard interval, is added to each of theparallel modulated waves at 204. This guard interval insures that thesubchannels will remain orthogonal, even if multipath fading causes thesubcarriers to arrive at the receiver with some delay spread. Theparallel streams with the cyclic prefix are then merged back into asingle serial stream at 204. Finally, the digital data stream isconverted to an analog signal 205, and output for wireless transmission.

The transmitted signal can be received by the receiver 210 and processedto recover the original data stream. First, the analog signal isconverted back to a digital signal by an analog to digital converter211. The cyclic prefix is removed and the separate subcarriers areconverted back to separate streams at 212. Each parallel data stream isdemodulated by a multicarrier demodulator 213, preferably with a FastFourier Transform algorithm. Finally, at 214 the parallel streams arereassembled into a single serial stream and output to the receivingdevice 215.

Any method may be used to make an initial selection of OFDM subchannelsto be used by the transmitter. One example method is illustrated in FIG.3.

FIG. 3 depicts a flowchart illustrating a process that can be utilizedby the transmitter to select subchannels to be used. This process couldbe implemented in hardware or software.

First, an application 301 requests a particular data rate fortransmission. This data rate would generally depend on the type of datato be transmitted, but for the purposes of this invention, any arbitrarydata rate could be requested.

At 302, the transmitter calculates the minimum number of OFDMsubchannels and maximum energy (or noise) threshold for each subchannelthat would be necessary to achieve the requested data rate.

The transmitter then begins an iterative process of selectingsubchannels to meet the required criteria. At 303, the transmitter tunesto one subchannel from within the spectral range available to it. At304, the transmitter detects the energy level on that channel. At 305,the transmitter compares the detected energy level with the thresholdfor that subchannel. If the energy level exceeds the threshold, thesubchannel is dropped 306. If it is below the threshold, the subchannelis kept 307.

The system then checks if it has identified a sufficient number ofsubchannels to meet the requirements at 308. If there are insufficientsubchannels, the system checks if there are more subchannels availablefor testing at 309. If other subchannels are available, the system willreturn to 303 and test the next available subchannel. If there are noother subchannels available, then the system will signal to theapplication that the requested data rate is not possible at 311.

Once the system has identified a sufficient number of subchannels, itwill then begin transmitting on those selected subchannels at 310. In apreferred embodiment, the Inverse Fourier Transform is performed acrossthe entire bandwidth used by the selected subchannels.

For example, the IEEE 802.11a standard provides for wirelesscommunications in the 5 GHz band of the spectrum. The available spectrumallowed for indoor use in the United States for the 802.11a standard isapproximately 5.180 GHz to 5.340 GHz, or 160 MHz wide. That 160 MHz ofspectrum is divided up into eight non-overlapping channels, each ofwhich is 20 MHz wide. Each 20 MHz channel may be divided up into 52subchannels according to OFDM principles, where each subchannel isapproximately 300 KHz wide. In this example there would thus be 416narrowband subchannels that could be used for transmission. To achievethe required data rate, the transmitter could select 20 subchannels thatdo not exceed a certain threshold for noise or interference. If thosesubchannels are spread across the first three 20 MHz channels, thetransmitter would perform an Inverse Fourier Transform algorithm on thesignals across that entire 60 MHz bandwidth. Note that the invention isnot limited to any part of the spectrum, any number of subchannels, orany standard for communication.

In an alternative embodiment of the invention, rather than checking theenergy level on each subchannel individually as depicted in FIG. 3, thesystem could check several subchannels at once, or detect energy on allof the subchannels in the entire available spectrum at one time, andthen discard the subchannels that exceed the energy threshold.

Various methods may be used by the receiver to determine the subchannelsin use. In one embodiment, the receiver performs the same energydetection as the transmitter to identify the correct subchannels. Inanother embodiment, the receiver receives a signal on a known frequencyfrom the transmitter indicating which subchannels have been selected fortransmission. In either of these embodiments, the receiver could performa conventional Fast Fourier Transform to recover the data. As with thetransmitter, the receiver would perform the Fast Fourier Transformacross the entire bandwidth used by all of the selected subchannels.

At this point, in one embodiment, the receiver may perform errorcorrection decoding on the received packets. The error correctiondecoder will first locate the errors, and use this information toidentify the subchannels causing the errors. In one embodiment, if thenumber of bits in error in a given subchannel exceeds a threshold, thenthe channel is labeled bad. As one example, the threshold could be50%—in other words, if more than 50% of the bits carried on a givensubchannel are bad, the channel itself is tagged as bad. Any thresholdmay be used, however. In another embodiment, rather than making a “hard”decision to tag a channel as bad at a given threshold, a “soft” decisionis made by incorporating several heuristics such as signal strength andother indicators of subchannel conditions.

The process of detecting and labeling subchannels is depicted in FIG. 4.In the depicted embodiment, the receiver receives and decodes packets400. In the process of performing error decoding on received packets,the receiver can identify and correct errors 401. Using the collectederror information, the receiver then identifies those subchannelscausing errors 402. The receiver may then use any algorithm to markcertain subchannels as bad based on detected errors 403. The receivermay also use additional metrics of subchannel condition such as signalstrength in step 403 to determine which channels to mark as bad.

Finally, the receiver may broadcast a vector indicating which channelsare bad channels and which channels are good channels 404. The sendermay receive the vector and use it to relocate transmissions to thebetter subchannels. Other clients may also use the bad channelinformation to make subchannel selection. In some embodiments, thereceiver repeats the error detection process at a regular interval andupdates the error vector so that new subchannels can be selected asconditions change. In other embodiments, the receiver repeatedlybroadcasts the error vector at a regular interval (with or withoutrepeating the checking process). For example, the error vector could bebroadcast every ten seconds.

In other embodiments, other transmit/receive pairs in the samegeographic area follow the same processes to select subchannels. Whentwo or more clients interfere by selecting the subchannel, each clientassociates a random timer with the subchannel with a timer widthdepending on the Quality of Service of the application beingtransmitted. When each client's timer runs out, the client can checkwhether the subchannel is in use. The first client to check thesubchannel should find no interference and can then claim thesubchannel. The second client, detecting continued interference, willthen relocate to another subchannel. This method can be extended to anynumber of clients in the same area.

In another embodiment, a transmitter listens to available subchannelsfor a certain period of time. The transmitter attempts to find a set ofsubchannels that (1) were not previously reported by its intendedreceiver as being bad, and (2) are not sensed to be bad by thetransmitter. If the transmitter identifies a sufficient number ofsubchannels that satisfy these conditions, the transmitter will transmitimmediately; otherwise, it will check back after a random time interval.In some embodiments, the random time interval can depend on the Qualityof Service requirements of the application being transmitted.

A general-purpose computing system will now be described, on whichembodiments of the invention may be implemented. With reference to FIG.5, an exemplary system for implementing embodiments of the inventionincludes a computing device, such as computing device 500, which may bea device suitable to function as a node of network environment.Computing device 500 may include at least one processing unit 502 andmemory 504. Depending on the exact configuration and type of computingdevice, memory 504 may be volatile (such as RAM), non-volatile (such asROM, flash memory, etc.) or some combination of the two. This most basicconfiguration is illustrated in FIG. 5 by dashed line 506. Additionally,device 500 may also have additional features/functionality. Memory 504is a form of computer-readable media that may store instructions, havingwireless medium parameters for various nodes in network environment.

Device 500 may include at least some form of computer readable media.Computer readable media can be any available media that can be accessedby device 500. By way of example, and not limitation, computer readablemedia may comprise computer storage media and communication media. Forexample, device 500 may also include additional storage (removableand/or non-removable) including, but not limited to, magnetic or opticaldisks or tape. Such additional storage is illustrated in FIG. 5 byremovable storage 508 and non-removable storage 510. Computer storagemedia includes volatile and nonvolatile, removable and non-removablemedia implemented in any method or technology for storage of informationsuch as computer readable instructions, data structures, program modulesor other data. Memory 504, removable storage 508 and non-removablestorage 510 are all examples of computer storage media. Computer storagemedia includes, but is not limited to, RAM, ROM, EEPROM, flash memory orother memory technology, CD-ROM, digital versatile disks (DVD) or otheroptical storage, magnetic cassettes, magnetic tape, magnetic diskstorage or other magnetic storage devices, or any other medium which canbe used to store the desired information and which can accessed bydevice 500. Any such computer storage media may be part of device 500.

Device 500 may also contain communications connection(s) 512 that allowthe device to communicate with other devices. Communicationsconnection(s) 512 is an example of communication media. Communicationmedia typically embodies computer readable instructions, datastructures, program modules or other data in a modulated data signalsuch as a carrier wave or other transport mechanism and includes anyinformation delivery media. The term “modulated data signal” means asignal that has one or more of its characteristics set or changed insuch a manner as to encode information in the signal. By way of example,and not limitation, communication media includes wired media such as awired network or direct-wired connection, and wireless media such asacoustic, RF, infrared and other wireless media. The term computerreadable media as used herein includes both storage media andcommunication media.

Device 500 may also have input device(s) 514 such as keyboard, mouse,pen, voice input device, touch input device, etc. Output device(s) 516such as a display, speakers, printer, etc. may also be included. Allthese devices are well know in the art and need not be discussed atlength here.

It should be appreciated that the invention is not limited to executingon any particular system or group of systems. For example, embodimentsof the invention may run on one device or on a combination of devices.Also, it should be appreciated that the invention is not limited to anyparticular architecture, network, or communication protocol.

Having now described some embodiments of the invention, it should beapparent to those skilled in the art that the foregoing is merelyillustrative and not limiting, having been presented by way of exampleonly. Numerous modifications and other embodiments are within the scopeof one of ordinary skill in the art and are contemplated as fallingwithin the scope of the invention. The foregoing description anddrawings are by way of example only. In particular, although many of theexamples presented herein involve specific combinations of method actsor system elements, it should be understood that those acts and thoseelements may be combined in other ways to accomplish the sameobjectives. Acts, elements and features discussed only in connectionwith one embodiment are not intended to be excluded from a similar rolein other embodiments.

Use of ordinal terms such as “first”, “second”, “third”, etc., in theclaims to modify a claim element does not by itself connote anypriority, precedence, or order of one claim element over another or thetemporal order in which acts of a method are performed, but are usedmerely as labels to distinguish one claim element having a certain namefrom another element having a same name (but for use of the ordinalterm) to distinguish the claim elements; The use of “including,”“comprising,” or “having,” “containing,” “involving,” and variationsthereof herein, is meant to encompass the items listed thereafter andequivalents thereof as well as additional items.

Having thus described several aspects of at least one embodiment of thisinvention, it is to be appreciated various alterations, modifications,and improvements will readily occur to those skilled in the art. Suchalterations, modifications, and improvements are intended to be part ofthis disclosure, and are intended to be within the spirit and scope ofthe invention. Accordingly, the foregoing description and drawings areby way of example only.

1. A method of wireless communication in an area comprising a pluralityof computing devices able to communicate wirelessly, the methodcomprising the acts, carried out by a first computing device, of: a)detecting, while receiving transmissions from a second computing device,transmission errors on a set of OFDM subchannels based on decodingreceived wireless communications using an error correction code, thereceived wireless communications being received with a receiver; b)identifying a plurality of undesirable subchannels from the set of OFDMsubchannels based at least in part on the errors detected in act (a);and c) broadcasting an identity of the plurality of undesirablesubchannels in a transmitted wireless communication such that theidentity of the plurality of undesirable subchannels is receivable by atleast one third computing device different from the first computingdevice and the second computing device.
 2. The method of claim 1,further comprising the act by the second computing device of receivingthe identity of the plurality of undesirable subchannels.
 3. The methodof claim 2, further comprising, on the second computing device,selecting OFDM subchannels for transmission based at least in part onthe received identity of the plurality of undesirable subchannels byselecting subchannels not in the received identity before selectingsubchannels in the received identity.
 4. The method of claim 1, whereinidentifying the plurality of undesirable subchannels comprisesidentifying subchannels of the set of OFDM subchannels in which morethan a predetermined percentage of transmitted bits are detected inerror.
 5. The method of claim 1, wherein the identifying the pluralityof undesirable subchannels further comprises identifying based on adetected signal strengths for each of the set of OFDM subchannels. 6.The method of claim 1, further comprises repeating the detecting,identifying, and broadcasting periodically.
 7. The method of claim 6,wherein the repeating comprises repeating according to a predeterminedtime interval.
 8. The method of claim 1, further comprising, on the atleast one third computing device, selecting OFDM subchannels fortransmission based at least in part on the received identity of theplurality of undesirable subchannels by selecting subchannels not in thereceived identity before selecting subchannels in the received identity.9. A wireless receiver for receiving OFDM transmissions in an areacomprising a plurality of computing devices able to communicatewirelessly, the receiver comprising: at least one processor programmedto act as: an error detection module for determining, whilecommunicating with a transmitter, transmission errors on a set of OFDMsubchannels based on decoding received wireless communications using anerror correction code; an identification module for identifying lowfidelity subchannels from the set of OFDM subchannels based at least inpart on the determined transmission errors; and a broadcasting modulefor broadcasting a vector representing the low fidelity subchannels suchthat the vector is receivable by at least one computing device differentfrom the transmitter and the receiver.
 10. The receiver of claim 9,wherein the identification module uses a hard decision model to identifylow fidelity subchannels.
 11. The receiver of claim 10, wherein theidentification module identifies subchannels as low fidelity if theerror detection module detects more than a predetermined percentage oferrors.
 12. The receiver of claim 9, wherein the identification moduleuses a soft decision model to identify low fidelity subchannels.
 13. Thereceiver of claim 9, wherein the broadcasting module broadcasts a vectorrepresenting the low fidelity subchannels at a programmable frequency.14. A system comprising the wireless receiver of claim 9, thetransmitter, and the at least one computing device different from thetransmitter and receiver, wherein the at least one computing devicecomprises at least one processor programmed to receive the vector fromthe wireless receiver and to select OFDM subchannels for transmissionbased at least in part on the vector by selecting subchannels not in thevector before selecting subchannels in the vector.
 15. Acomputer-readable storage medium having computer-readable signals storedthereon that define instructions that, as a result of being executed bya computer, instruct the computer to perform a method of wirelesscommunication in an area comprising a plurality of computing devicesable to communicate wirelessly, the method comprising acts carried outby a first computing device of: a) receiving data sent by a secondcomputing device across an OFDM spectrum; b) performing error detectionon the received data by decoding received wireless communications usingan error correction code; c) identifying OFDM subchannels in whicherrors were detected, based on the error detection in b); and d)advertising, to the plurality of computing devices, the OFDM subchannelsin which errors were detected.
 16. A computer-readable storage medium ofclaim 15, wherein the method further comprises detecting interferencefrom other transmitters.
 17. A computer-readable storage medium of claim16, wherein the method further comprises pausing data transmission for arandom time interval when interference from other transmitters isdetected.
 18. A computer-readable storage medium of claim 16, whereinthe method further comprises the following steps when interference isdetected from other transmitters: a) if a sufficient number ofnon-interfering error-free subchannels are available, transmitting onthe non-interfering error-free subchannels; and b) if a sufficientnumber of non-interfering error-free subchannels are not available,pausing data transmission for a random time interval before repeatingthe steps of claim
 15. 19. A computer-readable storage medium of claim15, wherein advertising the OFDM subchannels in which errors weredetected comprises advertising the OFDM subchannels in which more thanfifty percent of transmissions are errors.
 20. A computer-readablestorage medium of claim 15, wherein the step comprising receiving datasent across an OFDM spectrum is performed according to the IEEE802.11(a), IEEE 802.11(b), or IEEE 802.11(g) standards.